Time-lapse seismic surveying or monitoring involves obtaining seismic data of the same part of the subterranean formation at different times. It allows studying the changes in seismic properties of the formation as a function of time due to for example fluid flow through the underground formation, spatial and temporal variation in fluid saturation, pressure and temperature. Seismic data can be combined to generate images that show the changes. The time-lapse seismic surveying technique has applications such as mapping bypassed oil, monitoring injected reservoir fluids such as water, steam and CO2, and estimating fluid-flow heterogeneity related to pressure compartmentalization, and the hydraulic properties of faults and fractures. Time-lapse seismic surveying is also called 4-dimensional seismic surveying.
In the following discussion, the term “signal” is used to refer to raw, unprocessed data as well as to processed data, and the term “subsurface signal” is used to refer to a signal that contains information from the subsurface only, and does not include surface multiple reflections. This subsurface signal is also called the primary signal. The target layer in the subsurface is a layer or a number of layers in which one is interested. In the latter case, the layers can be separated by layers that are not of interest.
It is well known that repeatability is the key to the success of time-lapse seismic surveying, and that major concerns are source and receiver locations (see for example the article C P Ross and M S Altan, Time-lapse Seismic Monitoring: Some Shortcomings in Non-Uniform Processing, The Leading Edge, June 1997). Other concerns are source and receiver waveform responses and coupling, and various forms of undesired noise. Other factors that adversely affect the accuracy of time-lapse surveys in a marine environment are the changes in the sea state and properties of the sea.
In Applicant's co-pending International patent application publication No. WO02/075 363 discusses the effect of so-called surface multiple reflections. The surface multiple reflections are contributions to the recorded signal caused by multiple reflections of the seismic energy between the surface of the sea and the sea bottom. In this patent application it is explained that the surface multiple reflections do not repeat, so that when subtracting two signals, recorded at different dates, the difference signal contains an unknown contribution from the difference of the surface multiple reflections. Therefore this difference may not be the same as the difference between the subsurface signals (that are free from surface multiple reflections) and thus the difference is not an indication of changes in the subsurface between the date of recording the first signal and the date of recording the second signal.
In order to provide a method of carrying out at sea a time-lapse survey of a target layer in an underground formation in which non-repeatable effects caused by the surface multiple reflections can be suppressed in a simple manner, it is proposed in Applicant's International patent application to record the signals at substantially repeating sea conditions.